1. Field of the Invention
The present invention relates generally to lighting systems, and more specifically to luminaires for uniform intensity area lighting with controlled side glare.
2. Description of the Prior Art
Light intrusion has become an important concept in the lighting industry. Light from lamps must now be controlled so that only the area meant to be illuminated is illuminated by the lamp and outside area is not. For example, in a perfect street lamp implementation, no glow from a lamp should be visible from above or the sides of the designated area. Diffraction and scatter in prior art luminaires have allowed light, and thus the power to make the light, to be wasted by the inability to avoid direct light rays out the sides. Ordinances and industry practices frequently exist to prevent such side emissions for various reasons, for example, to make driving safer and to save on energy costs.
An improved roadway luminaire is described by the present inventor, Martin L. Lasker, in U.S. Pat. 4,651,260, (Lasker '260), issued Mar. 17, 1987. A source of light is reflected out from the luminaire by side-by-side reflectors, one of which is to one side of the light source. The light from the other reflector is adjusted to reinforce light coming from the second reflector in a preferred direction. Some prior art optical systems use a first reflector which surrounds a horizontal lamp in a standard Type II-III (roadway), "sharp cutoff" configuration. In Lasker '260, a part of this reflector causes an overly intense "hot spot", that increases in intensity as the angles of light move closer to the nadir at the base of the mounting pole. A subsequent redirection of the light escaping through this opening is accomplished by a second reflector which distributes otherwise wasted light at higher, more productive angles. The contour of the second reflector is such that no light above the cutoff angle of the first reflector occurs, so that the "sharp cutoff" of glare is retained. At least one lighting system described in the prior art uses a somewhat different approach to produce a Type V distribution with a vertical lamp, but both systems have the same basic problem. The high angle distribution from the second reflector has to pass through a clear side wall to keep the fixture within a practical size. Due to natural imperfections in the specular surface of the second reflector and in the material of the clear side walls, the luminaire appears to glow, despite is sharp cutoff of glare. It emits light at all angles, even those above 90.degree. from nadir.
FIGS. 1 and 2 illustrate a pair of light distribution patterns 10 and 12, shown in plan view, for the industry standard classifications "type V" and "square type V", respectively. Both have a luminaire 14 and 16 positioned in the center of the light-pattern. The square type V distribution 12 bulges out at the corners to fill a more rectangular pattern.
FIG. 3 shows the light distribution pattern 12 superimposed-over a grid of squares each having a side equal to one "mounting height" (MH), where one mounting height is the height of a luminaire above ground. Pattern 12 extends out three mounting heights on all four sides of the luminaire, e.g., where the luminaire is mounted thirty feet above the ground and three mounting heights would be ninety feet. The luminaire 16 is represented with an oblong bar positioned as with an arc tube of a horizontally held lamp.
FIG. 4 shows four illuminated areas, such as pattern 12, abutting each other as is typical in a typical large parking lot application.
FIG. 5 is an elevation view of the luminaire 16 and a pole 18 beneath it. Vertical angles from nadir (Point N) are shown that are required to reach points A, B, C and V, where "V" includes a plurality of points V beyond point C, as in FIG. 3. An important function of an optical system in an area lighting application permits the widest spacing between poles while not exceeding a ratio of ten to one between the brightest and dimmest points on the ground. Such a ratio is considered to define the bounds of uniform illumination. A good optical system should also sharply reduce the intensity of light at angles above 80.degree. to eliminate glare that can impede vision, light intrusion onto nearby properties and other forms of light pollution.
Lamps with an elongated arc tube, e.g., high pressure sodium and metal halide lamps used for area lighting, direct their greatest intensity of light perpendicular to the axis of the arc tube, a direction referred to as "broadside". The intensity becomes progressively weaker as the lamp (arc tube) is viewed from an on-end angle. The light falling on a point "A" in FIG. 5 comes from the broadside of the arc tube and therefore is at its greatest intensity. Not much additional light from the reflector system is required to effectively illuminate point A. A pair of points "B" and "B1" represent the weakest, on-end view of the lamp, therefore most of the light required at these points must come from a reflector system. A point "C", in FIG. 5, views the arc tube in an intermediate, foreshortened position, with a light intensity level between that at broadside and on-end.
However, point C is more distant from the luminaire 16, compared to points A and B, further diminishing the light level. As a result a larger amount of reflected light is required at point C for effective illumination. In order to strike the ground at point C, light must be projected at a vertical angle of 77.degree., only a few degrees below the angle at which a viewer standing at a point "V" should see relatively little light from the luminaire 16. This requirement is termed glare cutoff. At a point "N" (nadir), the arc tube broadside is closest. This is usually the maximum point of illumination, e.g., a hot-spot, against which a minimum point, such as points B or C, is compared for the 10:1 maximum/minimum relationship. It is therefore preferable that no light from the reflector be aimed at or near this point.
FIGS. 6 and 7 illustrate the performance of the two best performing systems designed for use in an opaque housing (that is, a luminaire housing which blocks light on all sides except the bottom aperture). A horizontal lamp system 30 has an arc tube 32 in a horizontal position. Because of this horizontal positioning, the tube 32 can be completely recessed in a housing 34 with a reflector 36 to provide a cutoff of direct light at angles above 80.degree., yet still be sufficiently exposed to provide direct light at angles below 80.degree.. System 30 has a few shortcomings that include a weak on-end and foreshortened views.
FIG. 8 illustrates the relative directions to points B and C (as in FIG. 5). These directions also face the surfaces of the reflector 36 and so the reflector too contributes to the illumination of these points. No matter how much light is redirected by the reflector 36, there is simply not enough cumulative intensity to provide an adequate level of illumination at all of the minimum points to allow very wide luminaire-to-luminaire spacing. A hot-spot at nadir also results from an intense direct broadside of the arc tube 32. Point N therefore receives an excessive accumulation of stray light from the generally wasteful surface of the reflector 36 above the lamp, because it also sees the broadside of the tube 32. For these reasons, the horizontal lamp system 30 cannot produce uniform illumination when spaced apart on a grid much more than five mounting heights by five mounting heights.
In FIG. 7, a vertical lamp system 40 has a broadside and on-end orientation which is better adapted to the desired distribution. However, the vertical length of the arc tube works against the system in other ways. In order to expose a sufficient portion of the arc tube so that it shines directly on point C, the lamp has to be dropped below the aperture. This position creates excessive brightness above 80.degree. both directly from the arc tube and from unavoidable reflections. The only way to restore glare control is to raise the lamp back into the housing where it can no longer produce a wide throw of light. Because of this the vertical lamp system is commonly offered for sale in two mutually exclusive configurations; either for wide spacing (six mounting heights by six mounting heights) without glare controls, or for glare control with much narrower spacing capability.
Two United States Patents describe devices for solving the problem of providing both uniform illumination on wide spacing and glare control, the vertical lamp described by Contra/Cline in U.S. Pat. No. 4,096,555, and the horizontal lamp described by Strada in U.S. Pat. 4,651,260. Both of these systems require that some of the light be emitted through the sides of the luminaire enclosure and therefore can not work with an opaque housing. However, the Strada patent has a reflector configuration that is useful in an improved system.